1. Field of the Invention
The present invention relates to a pre-pit detecting apparatus for detecting a pre-pit signal, which carries pre-pit information such as address information and which is recorded in advance on an optical disc on which record information of video data, audio data and the like can be recorded later by a user.
2. Description of the Related Art
There is a CD-R (Compact Disc Recordable) as a WO (Write-Once) type of high density recording medium, onto which the record information data can be written just once. The CD-R is an optical record medium having a record capacity similar to that of a CD dedicated to a reproduction. Pre-information, such as address information required to search a position on the optical disc at which the record information is to be recorded, rotation control information used for a rotation control of the CD-R, such as a synchronization signal, are recorded by wobbling an information record track (i.e., a groove track or a land track) on which the record information is to be recorded, in a wave form based on a signal which is FM (Frequency Modulation)-modulated in accordance with the pre-information to be recorded, at the stage of pre-formatting when the CD-R is manufactured.
When the information is actually recorded onto the CD-R, a signal including a wobbling frequency component of the wobbled track (hereafter, referred to as a wobbling signal) is detected, and a standard clock for controlling the rotation of the CD-R is extracted therefrom. Then, a drive signal for controlling a rotation of a spindle motor of rotating the CD-R is generated on the basis of the extracted standard clock. Also, a clock signal for the recording operation is generated which includes the information of a timing synchronous with the rotation of the CD-R.
Moreover, the address information on the CD-R required to record the record information is obtained from the wobbling signal. A position at which the record information is to be recorded is detected in accordance with the obtained address information to thereby record the record information.
There is a DVD-R (DVD-Recordable) in which the record density is improved over the CD-R. In this DVD-R, an interval between the information record tracks adjacent to each other is substantially half of that of the CD-R, from a request of improving the record density. Thus, even if the pre-information is obtained from the conventionally-wobbled information record track on the DVD-R, the wobbling signals on the information record tracks adjacent to each other interfere with each other. Hence, there may be a case that the wobbling signal cannot be exactly detected.
Then, in a case of the DVD-R, the pre-information is recorded by wobbling the information record track (for example, the groove track) on the DVD-R at a frequency in accordance with the standard clock and additionally forming pre-pits corresponding to the pre-information on a guide track (for example, the land track), which is positioned between the information record tracks adjacent to each other and which guides a light beam to the information record track. Moreover, the pre-pits are substantially uniformly formed on an entire surface of the DVD-R so that the standard clock can be generated from the pre-pits as the occasion demands.
The pre-information is obtained by detecting such a pre-pit, and the exact rotation control and the record control are carried out in accordance with this pre-information.
By the way, the applicant knows one method of detecting the pre-pit, which is carried out as follows.
That is, as shown in FIG. 10, a gate signal, which is phase-synchronous with the wobbling signal obtained by emitting a light beam onto the information record track, is generated. Then, only the signal exceeding a predetermined threshold signal is extracted by comparing the wobbling signal with the predetermined threshold signal.
Then, the pre-pit signal superimposed on the wobbling signal is extracted by calculating a logical product of this extracted signal and the gate signal.
However, if a defect resulting from a crack, a dust and the like is present on the DVD-R, there may be a case that this defect causes a noise in a form of a pulse, which is superimposed on the wobbling signal. In such a case, the pre-pit detecting apparatus using the above-mentioned method of detecting the pre-pit may erroneously detect the noise in the form of the pulse as the pre-pit signal.